New Zealand fishermen have caught what is believed to be the biggest specimen of a colossal squid, one of the most mysterious creatures on Earth.

The adult male weighs around 450kg (71 stone) and has eyes the size of dinner plates. Witnesses claimed that if calamari rings were made from it, they would be the size of tractor tyres.

Known by the scientific name Mesonychoteuthis hamiltoni, colossal squid are rarely seen intact. The new specimen will give scientists a rare insight into the creature's lifestyle and eating habits.

The only other complete specimen - a 300kg immature female - was caught on the surface in the Ross Sea near the Antarctic coast in April 2004.

The fishing crew who hauled in the new specimen were on a trip to catch Patagonian toothfish in Antarctic waters south of New Zealand when they ran into the squid.

"I can assure you that this is going to draw phenomenal interest - it is truly amazing," Steve O'Shea, a squid expert at the Auckland University of Technology, told the Associated Press.

Jonathan Ablett, of the Natural History Museum in London, said that scientists know very little about colossal squid.

"No one's ever seen one alive, apart from as it dies at the surface of the water. It's been shown they eat big fish and smaller squid, much like the giant squid does. There are so few specimens of any quality that this specimen is going to be an important addition."

Colossal squid are aggressive predators, killing their prey at depths of 2,000 metres.

Ideas about the way the squid moves and where it lives come from examination of remains in museums around the world and comparison with other squid species, such as the giant squid, Architeuthis.

Last year, the Natural History Museum put the most complete specimen of a giant squid on display. "No one's ever found a fully grown one and, in the past, people have exaggerated the total length that they get to," said Dr Ablett.

"People thought they might get up to 16 metres - possible they get even bigger. The one thing you notice is that, whereas with the giant squid, most of the length is made up with the tentacles, in the colossal squid it's the mantle [body] which is far bigger than the tentacles. The eye is even bigger."

Another theory suggests that colossal squid have the biggest single eyes on Earth but, because the only specimens of fully-grown squid are bits of body or tentacle, no one has been able to check whether this is true before now.

The squid, said to be almost 10 metres long, was frozen en route to New Zealand's national museum, Te Papa, in Wellington, where it will be preserved for scientific study.

Dr Ablett says that the new specimen will give scientists a better sense of the scale of the animal.

"Hopefully the stomach should be complete, so we'll have a better idea of what they might feed on," he said.

DNA work will be carried out on the squid to compare it with other specimens in collections around the world and give an understanding of the variability within the species.

Scientists will also be able to do a chemical analysis of the body to work out where the squid has spent its life. Different parts of the oceans have different chemical profiles and these can leave telltale signatures in the squid's tissues.

"We tend to only bump into giant and colossal squids accidentally when we go into their environment - we don't really know where they live or if that changes through a 24-hour period," said Dr Ablett.

He added that he was a little jealous of the New Zealand scientists who had been landed with the squid. "It would be an amazing specimen to work on."